1. Field of the Invention
The present invention relates to a control device for a printing apparatus, a control method, and a storage medium, and more particularly to time-division driving of a plurality of print heads and distribution of print data to the plurality of print heads.
2. Description of the Related Art
Printing apparatuses such as printers or copiers are configured to print images (including letters, symbols, and the like) on print media such as paper by using color material based on print information. The printing apparatuses are classified into an ink jet type, a wire dot type, a thermal type, an electrophotography type (a laser exposure type and an LED exposure type), and the like according to their printing systems. Of these types, a printing apparatus of an ink jet type (an ink jet printing apparatus) performs printing by using an ink jet print head to eject ink droplets from ejection ports of the print head toward a print medium.
Printing methods for the ink jet printing apparatus fall roughly into two types: a multipass system and a full line system. The multipass system repeats the operation of conveying a print medium by a predetermined amount in a direction crossing a direction in which a print head scans the print medium to perform printing over the entire area of the print medium. In the multipass system, the print head performs print scanning multiple times with respect to the same area on the print medium to print an image on the print medium. This can achieve a high image quality at relatively low cost. Therefore, ink jet printing apparatuses for consumers often use this system.
In the full line system, a print head has a printing width corresponding to the width of a print medium, and a print medium is moved so that an image is printed on the print medium. In this case, the print head performs print scanning once on the print medium. Such a print head having a long length and used in the full line system generally often has a configuration in which a plurality of print chips having a short length are arranged in a printing width direction. As compared to the ink jet printing apparatus of the multipass system, the cost of the apparatus body is higher, but it is possible to obtain an output product of a high image quality at high speed. Therefore, the full line system is often used in the ink jet printing apparatuses for POD (Print on Demand) or the like. Today, there is a need for high speed printing of print materials of a high image quality equivalent to that in offset printing, for example, at a high resolution of 1200 dpi×1200 dpi or greater, at a rate of several hundreds of pages to several thousands of pages per minute on a print medium having a Kiku size (152 mm×218 mm). Such a printing apparatus of the full line system is disclosed in, for example, Japanese Patent Laid-Open No. 2002-292859.
For a print head mounted on the printing apparatus of the full line system, a so-called multi-array head is often used, in which a plurality of arrays of printing elements that can print the same color material are arranged in parallel. Providing a plurality of printing element arrays associated with the same color material can print image data associated with a specific color material by a plurality of printing elements. This can suppress degradation in image quality caused by variations in landing positions of dots formed by ink droplets from respective printing elements or by variations in ejection amounts. Furthermore, since a time difference can be made between landings of adjacent dots on the print medium, it is possible to suppress degradation in image quality caused when dots which have landed on the print medium coalesce into one to form an inappropriate shape.
Each of the printing elements provided for the plurality of printing element arrays generally has a system using an electrothermal transducer element (heater) or a system using a piezoelectric element. Both systems control ejection of ink droplets by electric signals.
Printing elements in printing element arrays are arranged at a high density of, for example, 600 dpi. To downsize power sources for driving heads and members for power sources such as connectors and cables, the printing elements are often driven by a time-division driving system. In the time-division driving system, a plurality of printing elements are divided into sections, each including a predetermined number of printing elements. Then, each section is segmented into a plurality of driving blocks and the printing elements for each driving block are divided by time to be driven.
With reference to the attached drawings, the case of driving a print head by the time-division driving system will be described in detail.
FIG. 1 is a schematic view of ejection port arrays of a print head, driving signals for ejection ports, and ink droplets ejected from the ejection ports. In FIG. 1, an ejection port array 1 of a print head consists of 32 ejection ports, for example, and these ejection ports are divided into four sections, each section including eight ejection ports. Furthermore, each of the eight ejection ports in each section belongs to one of eight driving blocks, and is time-division driven for each driving block in printing. More specifically, ejection ports belonging to the same driving block in different sections are simultaneously driven.
In an example shown in FIG. 1, the number of segments is 8, and ejection ports are periodically assigned to one of the driving blocks, for example, four ejection ports (1st, 9th, 17th, and 25th ejection ports) in the ejection port array 1 to a first driving block, and another four ejection ports (2nd, 10th, 18th, and 26th ejection ports) to an eighth driving block. Then, the ejection ports from the first driving block to the eighth driving block are sequentially driven by pulse driving signals as shown in FIG. 1, and ink droplets 3 as shown in FIG. 1 are ejected from the respective ejection ports in response to the driving signals.
Time-division driving by pulse driving signals 2 with a time difference in ejection timings of ink droplets between driving blocks causes ink droplets to be ejected at different timings as shown in FIG. 1. Therefore, a time difference also occurs between timings at which dots by the ink droplets land on the print medium. As a result, dots shift from their ideal landing positions, and image quality may degrade. In particular, a thin line in a direction along an ejection port array direction may be misaligned due to variations in driving timings, and deterioration in image quality may easily be recognized.
As described, the technique of solving the problem of ragged lines is disclosed, for example, in Japanese Patent Laid-Open Nos. 2007-276353 and 2007-090714.
Japanese Patent Laid-Open No. 2007-276353 discloses a printing method in which the number of blocks driven in a single printing element array is reduced to 1/N and portions to be printed by the printing elements in a non-driven block are assigned to another print pass or another ejection port array for the same ink color. In this method, a time difference of block driving occurring in a single printing element array is reduced to 1/N, so as to reduce a shift of a landing position from an ideal position.
Further, Japanese Patent Laid-Open No. 2007-090714 discloses a driving method in which a plurality of printing element arrays are driven in different block driving orders. This method can reduce raggedness of lines.
Today, however, in ink jet printing apparatuses for POD printing or the like, there is an increasing need for high-definition output of image data, like offset printing. The above-described related art can reduce raggedness of thin lines, but a further improvement is required for such a need.
With respect to such a need, the method disclosed in Japanese Patent Laid-Open No. 2007-276353 can reduce raggedness of vertical lines, but variations in positions caused by a time difference in block driving after restriction still remain, and thus it cannot be said that the raggedness can be sufficiently reduced. As N increases, load on processing to another pass (another ejection port array in the full line system) and the number of passes increase. Therefore, it is difficult to indiscriminately increase a value of N in actuality.
With respect to such a need, the method disclosed in Japanese Patent Laid-Open No. 2007-090714 can reduce raggedness of vertical lines like Japanese Patent Laid-Open No. 2007-276353, but raggedness of thin lines still remains since block driving orders are different between printing element arrays.